ExecutorScheduler throws exception for the delayed action.

…edulers-merge

Conflicts:
	rxjava-core/src/main/java/rx/Observable.java

For now keeping ScheduledObserver an implementation detail until it's clear we want it part of the long-term public API.

ScheduledExecutorScheduler is just an extension of ExecutorScheduler so keeping them together for less surface area on the API.

Until there is a use case other than unit testing I'm moving this to a non-public role so it's not part of the public API.

- added Javadocs
- moved some classes to package-private until they are proven necessary for the public API
- made ExecutorScheduler support Executor implementations and still work with time delays by using a system-wide scheduler/timer
- made IO thread-pool unbounded with a cached thread pool

- I plan on using this to expand unit testing around various aspects of schedulers
- this is not done as an inner-class as it does not correlate with just one class but is cross-functional over many classes thus it fits best here

- the list of operators to add overloads to was derived from the Rx.Net docs at http://msdn.microsoft.com/en-us/library/hh212048(v=vs.103).aspx

- This applies to any pools RxJava itself creates. It will be up to users to do this for Executors they inject.

 … considering very long running app with lots of IO events.

I have some outstanding questions on how these should be implemented (or even why we need them when the 'subscribeOn' operator is far cleaner) so want to remove them for now so they don't make it into the public incorrectly.